In fields such as colored paints and conductive paints, there have conventionally been used dispersions of metal ultrafine particles, but there has not yet been proposed any technique for forming an electrode for the FPD, which makes use of such a dispersion of metal ultrafine particles as ink and the ink jet printing system. The FPD includes a variety of display systems such as a liquid crystal display (LCD), a plasma display panel (hereunder also referred to as “PDP”), an organic EL display (EL) and a field-emission-display (FED), but the PDP will hereunder be described as a typical example thereof in the present invention.
The PDP has become of interest lately as a large-scale display for consumer products, but there has been desired the development of a technique, which permits the simplification of the production process for the PDP to achieve a considerable decrease in cost and to thus popularize the PDP worldwide. First of all, the production process thereof will be described below while taking, as an example, the production of a color PDP for 42-inch high vision. This production process comprises the steps of preparing two kinds of panels of a front panel and a back panel. The electrode for the front panel is referred to as “a scanning electrode” and ITO transparent electrodes are formed on a glass plate at a density of 2 linear electrodes per one picture element for 1024 picture elements. The resistance value of the transparent electrode in itself is too high and thus a bus electrode (metal) is in general formed on the transparent electrode. The bus electrode has a width of 50 μm and a thickness of 2 μm and it has been formed, in the conventional electrode production process, according to the screen printing method in which a bus electrode is formed as a thick layer of an Ag paste or an electrode pattern-forming technique in which a film is formed on the entire surface by the sputtering technique and then an electrode pattern is formed by the photolithography technique using a resist film. On the other hand, the electrode for the back panel is called “an address electrode” and it is directly formed on a glass plate at a density of 3 linear electrodes per one picture element for 1024 picture elements. The address electrode has a width of 50 μm and a thickness of 2 μm and it is formed according to the screen printing method or the sputtering-photolithography technique like the scanning electrode. A dielectric glass layer is formed on the both scanning and address electrodes.
Both of the front and back panels are subjected to subsequent steps commonly employed and then bonded together to thus complete a PDP panel. Among the steps required for the production of such a PDP panel, the steps for forming the electrodes are the most complicated ones and require the use of a large number of steps. This becomes a major obstacle in the reduction of the production cost.
As methods for preparing the foregoing dispersion of metal ultrafine particles, there have been known, for instance, those that comprise the steps of blending metal ultrafine particles or powder with a solvent, a resin and/or a dispersant and then dispersing the particles by, for instance, stirring the mixture, applying of ultrasonics thereto, and treating of the mixture with, for instance, a ball mill or sand mill to thus give a desired dispersion of metal ultrafine particles. The dispersions prepared according to these methods have been used in the fields of, for instance, paints and the like. For instance, there have been known those for directly preparing metal ultrafine particles in a vapor phase such as a vapor phase-evaporation method (Japanese Patent No. 2,561,537) in which a metal is evaporated in a vapor phase in the coexistence of solvent vapor, the evaporated metal is condensed into uniform ultrafine particles to disperse them in the solvent and to thus form a dispersion and those, which make use of an insoluble precipitate-forming reaction or a reducing reaction using a reducing agent. Among these methods for preparing a dispersion of metal ultrafine particles, the vapor phase-evaporation method permits the stable preparation of a dispersion in which ultrafine particles having a particle size of not more than 100 nm are uniformly dispersed and requires the use of a dispersion stabilizer or a resin component in an amount smaller than that required for the liquid phase-production methods in order to prepare such a dispersion of metal ultrafine particles having a predetermined concentration.
As has been discussed above, the dispersion of metal ultrafine particles has never been used as the ink for ink jet printer or ink jet printing (system). This is because the conventional dispersions of metal ultrafine particles did not have sufficient characteristic properties (such as viscosity and surface tension) required for making the dispersions usable in the ink for the ink jet printing. The metal ultrafine particles produced according to the conventional vapor phase-evaporation method undergo coalescence. Therefore, if they are dispersed in a solvent, they never provide any stable dispersion. For this reason, when such a dispersion of metal ultrafine particles is used as ink for the inkjet printing, a problem arises such that aggregates of metal ultrafine particles present therein would cause clogging of the nozzle for inkjet printing. Moreover, even in a dispersion comprising metal ultrafine particles independently dispersed therein, it is required for the preparation of such a dispersion to use a solvent that can satisfy the requirements for ink or possesses the required ink-characteristics, but it has been quite difficult to select such an appropriate solvent.
Moreover, in the conventional vapor phase-evaporation method, the coexisting solvent forms by-products due to the denaturation or modification thereof upon the condensation of the metal vapor and accordingly, the method sometimes suffers from such problems as those concerning the storage life, stability with time, viscosity and coloration of the resulting dispersion, depending on the amount of the by-products. In addition, as will be detailed below, it is necessary to prepare a dispersion of metal ultrafine particles in, for instance, a low boiling point solvent, water and alcoholic solvents, the use of which is difficult in the steps of the vapor phase-evaporation method, depending of the applications of the resulting dispersion.
According to the conventional method for the production of PDP, the front panel and the back panel are produced in separate steps and they are ultimately assembled to give a PDP panel.
The steps for producing the front panel will first be described below. After a test for examining acceptability of glass plates, an ITO pattern serving as scanning electrodes is formed by the sputtering and photolithography techniques. Since the ITO film in itself has a high resistance value, a metal film having a width of 50 μm and a thickness of 2 μm is formed as a bus electrode. At present, there have been known two methods for forming such a metal film, i.e., the screen printing method for forming a thick Ag paste layer and a method comprising the step of etching a Cr/Cu/Cr laminated sputter film into a pattern according to the photolithography technique. After the formation of the bus electrode, a dielectric glass layer and an MgO layer are formed on the bus electrode pattern in this order and the front panel is thus ready for the assembly with the back panel.
Next, the steps for producing the back panel will be described below. After a test for examining acceptability of glass plates, an address electrode is formed. As methods for producing the address electrode, there have been known two methods as in case of the production of the front panel, i.e., the screen printing method for forming a thick Ag paste layer and a method comprising the step of etching a Cr/Cu/Cr laminated sputter film into a pattern according to the photolithography technique. After the formation of the address electrode, a dielectric glass layer, a stripe barrier rib, and a layer of a fluorescent substance are formed on the address electrode in this order and the back panel is thus ready for the assembly with the front panel. In the assembling step, these two panels are tightly bonded together, followed by evacuation of the space between these panels, the enclosure of a gas therein and then an aging treatment to thus complete a PDP panel.
However, the screen printing method used in the foregoing electrode-forming process suffers from problems such as defects concerning the electrode-forming position due to any slippage of the screen, open defects due to insufficient patterning, which is in turn caused due to the clogging of the screen and loss of material due to the paste remaining on the screen. The sputtering-photolithography technique is a vacuum process and requires pattern-etching procedures according to the photolithography. Therefore, this technique requires the use of 6 to 7 steps such as sputtering, application of a resist, irradiation of patterned light, development, etching and resist-ashing steps and a problem also arises such that the loss in material is large since a film is formed on the whole surface of the plate.
Accordingly, an object of the present invention is to solve the foregoing problems associated with the conventional techniques and more specifically to provide a method for forming electrodes for FPD using ink for the inkjet printing, which consists of a dispersion of metal ultrafine particles independently dispersed therein and which is prepared in such a manner that the dispersion has sufficient characteristic properties required for use in the ink for the ink jet printing, and an ink jet printer.